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Flexor digitorum longus tendons

The flexor digitorum longus (FDL) tendons are the distal extensions of the flexor digitorum longus muscle, a deep posterior compartment muscle of the leg. The FDL muscle functions to flex the lateral four toes (digits II–V) and assists in plantarflexion of the foot at the ankle joint.

After originating in the leg, the muscle forms a slender tendon that enters the tarsal tunnel posterior to the medial malleolus, passes deep to the flexor retinaculum, and divides into four tendinous slips that course along the plantar surface of the foot. Each tendon passes through the fibrous digital sheath of the corresponding toe and inserts on the base of the distal phalanx.

These tendons play a critical role in toe grip, push-off, and balance during walking and running.

Synonyms

  • Long flexor tendons of the toes

  • FDL tendons

  • Plantar digital flexor tendons

Origin, Course, and Insertion

  • Origin (muscle): Posterior surface of the tibia, distal to the soleal line and medial to the tibialis posterior origin

  • Course:

    • Descends posteriorly in the deep compartment of the leg

    • Passes posterior to the medial malleolus within the tarsal tunnel beneath the flexor retinaculum

    • Crosses superficially over the flexor hallucis longus tendon near the sustentaculum tali (known as the “knot of Henry”)

    • Divides into four tendinous slips for the lateral four toes

  • Insertion: Base of the distal phalanges of the second to fifth toes (plantar aspect)

Tendon Attachments

  • Each tendon passes through the digital fibrous sheath and the tendinous split of the flexor digitorum brevis before inserting distally

  • The tendons are enclosed in synovial sheaths that reduce friction and facilitate smooth gliding

  • The quadratus plantae muscle inserts into the lateral margin of the tendons, correcting the oblique pull of the FDL during toe flexion

Relations

  • Superiorly: Tibialis posterior muscle and tendon

  • Inferiorly: Flexor hallucis longus tendon (crossing at knot of Henry)

  • Medially: Tibia and posterior tibial nerve within tarsal tunnel

  • Laterally: Posterior tibial vessels and fibula

  • Anteriorly (plantar): Flexor digitorum brevis tendons in the sole

Nerve Supply

  • Tibial nerve (roots S1–S2)

Function

  • Toe flexion: Flexes the distal and proximal interphalangeal joints of the lateral four toes

  • Plantarflexion: Assists the gastrocnemius and soleus muscles in plantarflexing the foot at the ankle

  • Arch support: Contributes to maintaining the longitudinal arch of the foot

  • Stability and propulsion: Provides push-off power and enhances grip on uneven surfaces

Clinical Significance

  • Tendinopathy: Chronic overuse in runners or dancers leads to pain and thickening along the medial ankle or plantar surface

  • Tarsal tunnel syndrome: Inflammation of the FDL tendon sheath can compress the tibial nerve

  • Tendon tears or lacerations: May occur with penetrating trauma or hyperextension injuries

  • Post-surgical evaluation: MRI is essential for assessing tendon repair integrity and postoperative scarring

  • Subluxation or adhesions: May occur after chronic tendinopathy or trauma, restricting toe motion

MRI Appearance

  • T1-weighted images:

    • Tendons: low signal intensity (dark), uniform and sharply defined

    • Muscle belly: intermediate signal intensity

    • Fat around tendons: bright and provides excellent contrast

    • Chronic tendinopathy: fusiform thickening with slightly higher signal than normal

  • T2-weighted images:

    • Normal tendon: low signal intensity (black)

    • Normal muscle: intermediate signal, slightly darker than on T1

    • Tendinopathy or partial tear: bright focal hyperintense areas within or around tendon sheath

    • Tenosynovitis: fluid-filled sheath appears bright hyperintense surrounding tendon

  • STIR:

    • Normal tendon and muscle: intermediate-to-dark signal

    • Pathologic tendon: bright hyperintensity in areas of edema or fluid accumulation

    • Excellent for detecting inflammatory changes and early tendinitis

  • Proton Density Fat-Saturated (PD FS):

    • Normal tendon: uniformly dark, well-marginated

    • Normal muscle: intermediate-to-dark signal

    • Partial tears: focal bright signal clefts or peritendinous fluid

    • Tenosynovitis: diffuse bright signal along tendon sheath

  • T1 Fat-Sat Post-Contrast:

    • Normal tendon: minimal to no enhancement

    • Inflamed sheath: linear or circumferential enhancement

    • Partial tears: enhancing granulation tissue around tendon defect

    • Post-surgical scarring: irregular peripheral enhancement

CT Appearance

Non-Contrast CT:

  • Tendons: soft-tissue density, best visualized in the tarsal tunnel and plantar aspect

  • Chronic disease: tendon thickening, sheath calcification, or subtle ossification

  • Tenosynovitis: may show increased soft-tissue density around tendons

Post-Contrast CT (standard):

  • Tendons: enhance poorly (fibrous composition)

  • Surrounding inflamed soft tissues: enhance moderately

  • Useful for assessing postoperative changes, tendon sheath pathology, and osseous involvement

MRI image

Flexor digitorum longus tendons sfov of the Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Flexor digitorum longus tendons sfov of the Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

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Flexor digitorum longus tendons sfov of the Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00002

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Flexor digitorum longus tendons sfov of the Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00003

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Flexor digitorum longus tendons sfov of the Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00004

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Flexor digitorum longus tendons sfov of the Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00005

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Flexor digitorum longus tendons sfov of the Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00006

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Flexor digitorum longus tendons sfov of the Foot axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00007

CT image

Flexor digitorum longus tendons ct axial